Fracturing, or hydraulic fracturing, involves the injection of specialized fluids into geologic formations to create fractures. These fractures, in combination with flow paths drilled by wellbores, allow hydrocarbons, such as oil or natural gas, to flow from the deep recesses of the geologic formations to the ground surface. The injected specialized fluids are called fracturing fluids, which carry proppant into the fractures.
For many years, fracturing fluid technology has centered on a wide range of polymers. Polysaccharides such as guar, hydroxypropyl guar (HPG) and hydroxyethylcellulose (HEC) are the most commonly used polymers to provide the fluid viscosity. Since high viscosities are needed to carry the required amount of proppant, high concentrations of polymer are used and the polymers are frequently crosslinked to achieve the necessary viscosity. Cross-linked polymer gels, such as those based on guar crosslinked with borate or other polymers crosslinked with metal ions, are typically used to attain a sufficient fluid viscosity and thermal stability in high temperature reservoirs.
The crosslinking agents for polysaccharide solutions used in well treating fluids are well known in the art. One common type of crosslinker is transition metal containing compounds including titanium (4+), zirconium (4+), chromium (3+), iron (3+) and aluminum (3+) formulated as a product so as to release transition metal ions when dissolved in an aqueous liquid. The most commonly used crosslinker is based on borate releasing compounds due to its reversibility of crosslinking. The borate source may include easily soluble borate oxides such as boric acid and/or sodium borate (borax). The borate source can also be lowly soluble borate minerals such as ulexite, colemanite, probertite, and mixtures thereof. Products of this type have a technical advantage of delayed crosslinking action upon use, which is a desired property for many fracturing applications, and is a consequence of the low solubility. The borate source can also be a mixture of lowly soluble borate minerals and/or boric acid and/or ester derivatives of boric acid and/or borax and/or alkali borate salts and/or alkali earth borate salts.
In order to add borate or metallic crosslinkers easily and accurately into the fracturing fluids, it is better for the crosslinker to be in a liquid form. In many cases, it is advantageous to have highly concentrated formulations with actives present in excess of their solubility limits, typically dispersed as a suspension of the particular active in an appropriate liquid suspension medium. One common medium is non-aqueous liquids such as hydrocarbon solvents. See U.S. Pat. No. 6,024,170, which is incorporated fully herein by reference.
Numerous problems exist when utilizing non-aqueous suspensions. Many of the non-aqueous liquid suspensions are environmentally unacceptable due to the choice of the oil and have poorer suspension and stability characteristics, and may be more expensive and difficult to prepare.
US 2003/0144154 A1, which is fully incorporated herein by reference, discloses a concentrate for the crosslinking of polymers in aqueous solutions, which comprises an alkali metal formate solution, a crosslinking agent, a suspension agent, and optionally a deflocculant. The preferred alkaline metal formate is potassium formate. The preferred suspension agent is a clay mineral selected from the group consisting of smectite clays, palygorskite clays and mixtures thereof. The key of this technology is the addition of formate salt, which decreases the freezing point of the product and increases the density of the suspension. Under this technology, however, clay minerals are also required.
Solutions of low molecular weight quaternary ammonium compounds have also been used in hydraulic fracturing fluids for their clay stabilizing properties, in which the presence of this type of ion helps prevent the swelling of clay minerals present in the subterranean formation, which come into contact with water as a result of the fracturing operation. Clay swelling and associated fines migration leads to reservoir permeability damage. Tetramethyl ammonium chloride is the historically most used substance in this category, but the natural product choline in the form of its chloride salt has in recent years become the product of choice because of its inherently non-toxic and bio-assimilation properties.
A perhaps lesser appreciated property of choline chloride is the high water solubility (70-75 weight % are typical solution concentrations) and corresponding high solution density and low freezing point. This makes choline chloride solutions ideal media for suspension of dense materials such as sparingly soluble borate minerals and metal ions. Another feature of choline is the hydroxyl functionality, which will react with soluble borate species to yield borate esters. This enables the choline solutions to co-solubilize borate in high percentages and still to retain the desired pour point and pumpability of the overall fluid. Such solutions would advantageously combine multiple fuctions, i.e. clay stabilization and crosslinker actives, into one easily managed and deliverable formulation package.
Accordingly, there is a need for fracturing fluids that are readily hydratable and have controllable viscosity properties through crosslinking that enable efficient and inexpensive pumping and proppant transport down boreholes.